Unfold then spread ½ of the mushroom/onion mixture over the sheet. For a pretty design, roll up the pastry sheet from both ends, meeting in the center and then gently pressing together. You can also just roll the entire thing from one side, which is easier in my opinion.
This recipe makes about 3 dozen from two sheets, depending on how thinly you slice the Palmiers. The recipe is adapted from one by Susan LeTerneau.